1. Field of the Invention
The present invention relates to a method for improving transmission properties, such as a signal-to-noise ratio, in a return link in backscatter RFID or remote sensor systems, whereby at least one transponder or sensor transmits useful data in, for example, an asynchronous mode to at least one reader. Moreover, the invention also relates to an apparatus for wireless data transmission including at least one reader and at least one transponder or remote sensor being located in an electromagnetic field of the reader, whereby the reader is designed at least for sending and the transponder or sensor at least for receiving a header with transmission parameters.
2. Description of the Background Art
Automatic identification methods, also called auto-ID, have been widely used in recent years in many service sectors, in acquisition and distribution logistics, in commerce, in production, and material flow systems. The goal of auto-ID is, for example, the provision of information on persons, animals, objects, and products, etc.
An example of such auto-ID systems are chip cards, which are widely used today, in which a silicon memory chip, via mechanical-galvanic contacting using a reader, is provided with power, read out, and optionally can also be reprogrammed. In this case, the acquisition device is routinely called the reader, regardless of whether data can only be read thereby or also rewritten.
In RFID systems, the data carrier, e.g., the transponder, can be supplied with power not only through galvanic contact but also contactless through electromagnetic fields within the radio range (radio frequency: RF).
RFID systems typically have two basic components, namely, a transponder or a sensor in the case of a remote sensor system, i.e., an application-specific integrated circuit (IC) with a coupling element, such as a dipole antenna as a transmitter and receiver, and of a reader (also: base station), which typically has a high frequency module (transmitter-receiver) and also a coupling element. The reader provides the transponder or sensor, which usually does not have its own power supply, with power and a clock signal. Data is transmitted from both the reader to the transponder (forward link) and also in the opposite direction (return link). In this case, routinely before the start of the actual useful data transmission in the return link, a so-called return link header is transmitted by the transponder or sensor, which defines the transmission parameters of the return link, e.g., the modulation coding to be used or the like.
Such RFID systems, whose range is considerably greater than 1 m, work with electromagnetic waves in the UHF and microwave range. In this case, a backscattering method, called the backscatter principle because of its physical operating mode, is used predominantly, during the course of which a portion of the energy arriving at the transponder from the reader is reflected (backscattered) and in so doing is optionally modulated for data transmission. The IC receives, via the coupling element, a high frequency carrier, which it transmits by suitable modulation and backscattering devices partially back to the reader.
The RFID and remote sensor systems, outlined above and based on backscattering, generally have the disadvantage that the return link is very weak with respect to the power balance, primarily because of the free space attenuation both in the forward and return link. For this reason, attention must be focused especially in the design of such systems that a high signal-to-noise ratio (SNR) and thus a low bit error rate can be achieved.
A possible approach is the use of “synchronous return links,” in which the reader at certain time intervals sends synchronization tags (notch signals), which define a bit length in the return link. Thereby, additional expenditure for circuitry is usually necessary, which has an unfavorable effect on the price of such systems. Moreover, during use of synchronous return links an interfering effect on other readers in the vicinity due to the unfavorable power balance is disadvantageous.
ISO standard 18000-6 FDIS, furthermore, describes systems with an asynchronous return link, in which a transponder or sensor transmits a “free” data stream without being affected by synchronization tags sent by the reader. Such asynchronous link mechanisms can be realized less expensively in UHF RFID systems than the aforementioned synchronous link mechanisms. Moreover, asynchronous methods possess advantages during use in RFID or remote sensor systems, which comprise a plurality of readers within a common range, because the noise contribution can be reduced by asynchronous operation.
For efficient data transmission in systems operating asynchronously, it is of exceptional importance that the transponder/sensor and reader operate at the same data rate. ISO standard 18000-6 FCD for this purpose discloses on page 17, FIG. 8, a return link header, which is intended to enable a reader to synchronize to the data rate of a transponder. For this purpose, the header contains a specific sequence, uniform over time, of modulation states (on/off). The ANSI T6 standard (page 4-1, FIG. 4-1) shows another example of such an approach.
To keep the signal-to-noise ratio of the link tolerable, a relatively stable baud rate is necessary during asynchronous transmission. This is derived from an on-chip oscillator disposed in the transponder or sensor, which oscillator, however, routinely has manufacturing-related tolerances with respect to its oscillation periods. Nevertheless, to be able to achieve the values defined, for example, in ISO 18000-6 FDIS (page 10, input tag:9/tag:9a) (40 kbit/s with ±15% tolerance), additional circuit costs (use of different current reflectors) for the purposes of equalization are necessary in such prior-art solutions, because the expected process tolerances of free running RC oscillators are far greater.
Moreover, the tolerances in a routinely sought low-power design of oscillators increase. Conventional systems achieve the desired accuracy, which is necessary, among others, for detecting collisions in the protocol (more than one transponder or sensor sends a data stream to the reader) or to reduce the time windows in Aloha-based anticollison methods (see, e.g., Finkenzeller, RFID-Handbuch, 3rd ed., Hanser, pp. 290ff, which is herein incorporated by reference), by storing a correction value in a permanent read-only memory, whereby this value is then repeatedly read out preferably after a power-on-reset (POR) and supplied to a matching circuit (current reflector switching-over). A relative frequency accuracy can be maintained in this manner. The thus obtained oscillator clock signal when the return link is activated is fed to, e.g., a power splitter, which then, by suitable wiring, generates limit tags for the data transmission to the reader at the desired baud rate.
In the adjustment procedure of the aforementioned type, it is to be regarded as a particular disadvantage that storing additional bits uses scarce disk space. In addition, the aforementioned readout and adjustment procedures require time and cause additional activity (power consumption) on the transponder or sensor chip. Moreover, the necessary circuit measures require additional space and additional current, which has a negative effect both on the range of the link and also on the price of the transponder or sensor IC. Furthermore, an additionally possible temperature drift of the oscillator is generally not taken into account: area- and current-intensive measures are again required to compensate for this. As a result, in prior-art systems the baud rate is thereby not stable, so that very low signal-to-noise ratios result for certain applications.